1. Field of the Invention
This invention relates to a method for the production of methacrylic acid and more particularly to a method for the production of methacrylic acid, which includes a step of starting up a reactor by introducing therein a preheated gas so as to retain the relative humidity of the catalyst bed not more than 40% while elevating the temperature of the reactor.
2. Description of the Related Art
Methacrylic acid is the raw material for a general-purpose resin which demands quantity production. Heretofore, it has been known to adopt for the production of methacrylic acid a method using a shell-and-tube type reactor which has incorporated therein built-in reaction tubes packed with an oxidizing catalyst. The official gazette of JP-A-04-90853, for example, discloses a method for the production of methacrylic acid, which is characterized by using a methacrylic acid-producing catalyst containing at least the oxygen compounds of phosphorus and molybdenum. The official gazette of JP-A-2001-11010 discloses a method for producing methacrylic acid by effecting the gas phase catalytic oxidation of methacrolein with a fixed-bed shell-and-tube type reactor, which method comprises using a molybdovanadophosphoric acid complex oxide as a catalyst and requiring the reaction tubes to be packed with the catalyst in such a manner that the quantity of the catalytically active component per unit volume of the reaction tubes may decrease from the raw material gas inlet part toward the outlet part.
Methacrylic acid is produced by the oxidation and/or oxidodehydrogenation of at least one compound selected from among methacrolein, isobutyl aldehyde, isobutyric acid, and isobutane. This reaction is exothermic in kind. For the purpose of recovering the heat of this reaction, therefore, it is necessary to circulate a heat medium to the reactor. Since the solidifying point of the heat medium generally is so high as to fall in the range of 50-250° C., the production of methacrylic acid with a reactor which is left standing at room temperature necessitates adoption of a specific method of starting up the reactor. The official gazette of JP-A-2001-310123, for example, discloses as a means of starting up a reactor adapted to circulate such a heat medium, a method which effects quick startup of the reactor by introducing a gas heated to a temperature in the range of 100-400° C. to the reaction tubes' side thereby starting elevation of the temperature of the reaction tubes, then circulating a heated heat medium to the fluid outside the reaction tubes, and introducing a gas of an elevated temperature in advance to the reaction tubes thereby preventing the heat medium which has been circulated from being re-solidified.
Since methacrylic acid is the raw material for a general-purpose resin, it is required to have the production efficiency thereof further improved by exalting the conversion ratio and the selectivity of the raw material compound. Further, methacrylic acid is a compound which is produced by continuously feeding the raw material gas and operating the plant for a long time. Thus, it is required to develop the development a method for improving the service life of a catalyst for use in the production line thereby exalting the productivity of the pertinent operation and stabilizing the production of methacrylic acid for a long time.